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Abstract:

A photon detecting device including a sensor including a vacuum chamber,
a photocathode arranged therein to convert photons to primary electrons,
a converter converting at least part of the energy of accelerated primary
electrons to secondary charges collected by a plurality of detection
cells, an acquisition circuit adapted to read the charges collected by
the detection cells with an integration time allowing an impact density
to be obtained per unit of time and per unit surface of a cell of the
order of a single electron, a system identifying a cluster of adjacent
detection cells of which at least one so-called main cell includes a
quantity of collected charges higher than a threshold value, a system
determining at least one characteristic of the cluster, a system
memorizing at least one characteristic of a reference cluster resulting
from conversion of a primary electron, and a system comparing the
determined characteristic(s) of the cluster with the memorized
characteristic(s) of the reference cluster to evidence whether the
cluster results from the conversion of a primary electron.

Claims:

1. Device (1) for detecting photons (5), comprising a sensor (2)
including: a vacuum chamber (3), a photocathode (4) arranged in the
vacuum chamber (3), designed to convert the photons (5) to primary
electrons (6) and subjected to an electromagnetic field (E) adapted to
accelerate the primary electrons (6), a converter (8) converting
electrons (6) to secondary charges (9), arranged in the vacuum chamber
(3) facing the photocathode (4) and adapted to convert at least part of
the energy of the accelerated primary electrons (6) into secondary
charges (9) collected by a plurality of detection cells (10) set at a
regular pitch, the converter (8) optionally converting into secondary
charges (9) so-called parasitic electrons not derived directly from the
conversion of a photon (5) and possibly being of backscattered type, of
the type resulting from ion back bombardment or of the type derived from
electronic emission background noise, an acquisition circuit (13) adapted
to read the charges (9) collected by the detection cells (10) with an
integration time allowing an impact density to be obtained per unit of
time and per unit surface of a cell (10) of the order of a single
electron, said detection device (1) being characterized in that it
comprises: an identification system (14) identifying a cluster (15) of
adjacent detection cells (10) of which at least one so-called main cell
(10a) comprises a quantity of collected charges (9) higher than a
threshold value Vs, a determination system (19) determining at least one
characteristic of the cluster (15), a memory system (20) memorizing at
least one characteristic of a reference cluster (15a) resulting from the
conversion of a primary electron (6), a comparison system (21) comparing
the determined characteristic(s) of the cluster (15) with the memorized
characteristic(s) of the reference cluster (15a) in order to evidence
whether the cluster (15) results from the conversion of a primary
electron (6).

2. The device according to claim 1, characterized in that the
identification system (14) comprises: a system (17) for evidencing at
least the main cell (10a) whose quantity of collected charges (9) is
higher than the threshold value Vs, a recognition system (18) recognizing
the cluster (15), from the charges (9) collected in the cells (10b)
adjacent the main cell (10a).

3. The device according to claim 1, characterized in that it comprises an
elimination system (22) to eliminate a cluster (15) identified by the
comparison system (21) as not resulting from the conversion of a primary
electron.

4. The device according to claim 1, characterized in that: the memory
system (20) memorizes at least one characteristic of a standard cluster
(15b) resulting from the conversion of a backscattered electron (61), the
device (1) comprises a confrontation system (26) confronting the
determined characteristic(s) of the cluster (15) with the memorized
characteristic(s) of the standard cluster (15b), in order to evidence
whether the cluster (15) results from the conversion of a backscattered
electron (61); the device (1) comprises an elimination system (22) to
eliminate a cluster (15) evidenced by the confrontation system (26).

5. The device according to claim 1, characterized in that: the memory
system (20) memorizes at least one characteristic of a model cluster
(15c) resulting from ion back bombardment, the device (1) comprises a
similarity search system (28) between the determined characteristic(s) of
the cluster (15) and the memorized characteristic(s) of the model cluster
(15c) in order to evidence whether the cluster (15) results from ion back
bombardment, the device (1) comprises an elimination system (22) to
eliminate a cluster (15) evidenced by the similarity search system (28).

6. The device according to claim 1, characterized in that the device (1)
further comprises a system for generating a video signal (33) from
clusters (15) identified and non-eliminated over a give time interval.

7. The device according to claim 1, characterized in that the device (1)
further comprises: a determination system (29) determining the position
of the photon (5) at the origin of the each identified and non-eliminated
cluster (15), a storage system (30) storing at least part of all the
determined positions, a comparison system (31) comparing each new
determined position with the stored positions, an identification system
(32) using the result of the comparison of the new positions which are
not included in the stored positions, in order to evidence the positions
of electrons resulting from electronic emission background noise by
thermionic effect or by field effect, an elimination system (22)
eliminating positions evidenced by the identification system (32).

8. The device according to claim 7, characterized in that the device (1)
further comprises a system for generating a position signal (33) from
each determined, non-eliminated position.

9. A method for processing data produced by a sensor (2) of the type
including: a vacuum chamber (3), a photocathode (4) arranged in the
chamber (3), designed to convert photons (5) emitted by at least one
light source into primary electrons (6) and subjected to an
electromagnetic field (E) adapted to accelerate the primary electrons
(5), a converter (8) converting electrons (6) into secondary charges (9),
arranged in the chamber (3) facing the photocathode (4) and adapted to
convert at least part of the energy of the accelerated primary electrons
(6) into secondary charges (9) collected by a plurality of detection
cells (10) distributed at a regular pitch, the converter (8) optionally
converting to secondary charges (9) so-called parasitic electrons not
derived directly from the conversion of a photon (5) and possibly being
of backscattered type, of the type resulting from back bombardment or of
the type derived from electronic emission background noise, an
acquisition circuit (13) adapted to read the charges (9) collected by the
detection cells (10) with an integration time allowing an impact density
to be obtained per unit of time and per surface unit of a cell (10) of
the order of a single electron, characterized in that it comprises the
following steps repeated successively and continuously: identifying a
cluster (15) of adjacent detection cells (10) of which at least one
so-called main cell (10a) comprises a quantity of collected charges (9)
higher than a threshold value Vs, determining at least one characteristic
of the cluster (15), comparing the determined characteristic(s) of the
cluster (15) with at least one characteristic of a reference cluster
(15a) resulting from the conversion of a primary electron (6), in order
to evidence whether the cluster (15) results from the conversion of a
primary electron (6).

10. The processing method according to claim 9, characterized in that the
identification step of the cluster (15) comprises the following steps:
identifying at least the main cell (10a) whose quantity of collected
charges (9) is higher than the threshold value Vs, recognizing the
cluster (15) from the quantity of collected charges (9) in the cells
(10b) adjacent the main detection cell (10a).

11. The processing method according to claim 9, characterized in that the
method comprises a step to eliminate a cluster (15) identified as not
resulting from the conversion of a primary electron (6).

12. The processing method according to claim 9, characterized in that the
method comprises: a confrontation step between the determined
characteristic(s) of the cluster (15) and at least one characteristic of
a standard cluster (15b) resulting from the conversion of a backscattered
electron (6a), in order to identify whether the cluster (15) results from
the conversion of the backscattered electron (6a), a step to eliminate
clusters (15) evidenced at the confrontation step.

13. The processing method according to claim 9, characterized in that the
method comprises: a step confronting the determined characteristic(s) of
the cluster (15) with at least one characteristic of a model cluster
(15c) resulting from ion back bombardment, in order to identify whether
the cluster (15) results from ion back bombardment, a step to eliminate
clusters (15) evidenced at the confrontation step.

14. The processing method according to claim 9, characterized in that it
further comprises a step to generate a video signal from clusters (15)
identified and non-eliminated.

15. The processing method according to claim 9, characterized in that it
further comprises the following steps: determining the position of the
photon (5) at the origin of each cluster (15) identified and
non-eliminated, storing at least part of all the determined positions,
comparing each new determined position with the stored positions,
identifying, from the result of comparison, those new positions which are
not included in the stored positions, in order to evidence the new
positions resulting from electronic background noise, eliminating the new
positions evidenced at the identification step.

16. The processing method according to claim 15, characterized in that it
further comprises a step for generating a position signal from the
determined, non-eliminated positions.

17. The processing method according to claim 16, characterized in that,
from the step for generating a position signal, it consists of
determining the position of a single-photon point emitter per image and
optionally of tracking the position of this emitter.

Description:

[0001] The technical field of the invention concerns the detection of
photons in scenes with low light level.

[0002] More particularly, the invention concerns the field of photon
detection devices comprising a sensor having sensitivity for
single-photon detection, and their methods of implementation.

[0003] The invention finds preferred, but non-exclusive application in the
field of processing Night Vision scenes, for example in the area of
defence and security, or scientific and industrial imaging such as
fluorescence microscopy.

[0004] In the state of the art, for this purpose it is known to use an
Electro Bombarded Complementary Metal-Oxide Semiconductor, abbreviated to
EBCMOS.

[0005] Document WO 0106571 describes said sensor which is composed of a
photocathode and of an array of silicon pixels. These two components are
sealed and form a vacuum chamber.

[0006] The photocathode converts the photons emitted by at least one light
source into primary electrons, by photoelectric effect. The primary
electrons are accelerated in the direction of the pixel array by an
electromagnetic field which imparts sufficient kinetic energy to these
electrons so that they can be individually detected.

[0007] The pixel array comprises a detection volume, arranged facing the
photocathode, the accelerated primary electrons entering this volume. The
interaction of the primary electrons within the detection volume sets up
charges which diffuse as far as diodes arranged at a regular pitch, e.g.
in an array, and adapted to collect the charges whilst optimizing
read-out noise. The elementary diode unit corresponds to a pixel and it
is possible to have several diodes per pixel.

[0008] The accumulation of charges in the diodes allows the forming of
electric signals that are processed so as to generate a video signal
composed of a succession of images. Each image corresponds to the sum of
energies deposited in the pixels during an integration time of the
sensor.

[0009] This type of sensor was chiefly designed for use in scenes with low
light level, for example for night vision, with image frequencies of the
order of 25 Hz corresponding to an integration time of 40 ms.

[0010] However, the pixel array also converts electrons which are not
derived directly from the photocathode after conversion of a photon,
which is undesired. This generates parasitic effects in the image
recorded by the pixel array.

[0011] One first type of parasitic effect is the halo effect. This is due
to the backscattering of primary electrons on the pixel array, to their
re-acceleration in the direction of the pixel array and to detection
thereof by the array. The image effectively assumes the sum of the
energies deposited in the pixels during the integration time of the
sensor, whether this energy derives from backscattered electrons or from
primary electrons. For scenes with low light levels i.e. less than one
mLux, and containing a localized more intense source, an accumulation of
charges occurs due to the backscattered electrons, which leads to the
formation of a circular halo around the image of the source. The radius
of the halo is equal to no more than twice the distance between the
photocathode and the pixel array. All the images of the objects contained
in this halo disappear. With distances between the photocathode and the
pixel array usually of the order of one millimetre, the halo may cover a
large part of the sensor surface and the sensor loses its entire
function.

[0012] A second type of parasitic effect is the so-called effect of "ion
back bombardment at the photocathode". A residual atom present in the
vacuum chamber may be ionized by the primary electrons before it is
itself accelerated towards the photocathode. This is followed by
localized pulling of numerous electrons from the photocathode, these
being accelerated in the direction of the pixel array. This high density
of electrons translates as an intense point in the resulting image.

[0013] A third type of parasitic effect is the so-called "dark count"
effect. This effect results from electronic emission background noise
derived from the emission of electrons by the photocathode under
thermionic effect or field effect. For images of scenes with low light
level, electronic background noise is responsible for a random snow
effect in the reconstructed scenes.

[0014] To endeavour to reduce the phenomenon of electron backscattering
and hence the halo phenomenon, patent application US 2005/0122021
proposes that the electron collecting surface should have a textured
surface. In addition to the problem of manufacturing said surface, this
solution eliminates part of the sensitive collecting surface and does not
allow the parasitic effects in a recorded image to be reduced.

[0015] It is one objective of the invention to allow the detection or
recognition of photons in scenes with low light levels despite the
presence of parasitic effects.

[0016] A further objective of the invention is to propose a method for
processing data produced by a sensor adapted for the processing of scenes
with low light levels, which allows the detection of so-called primary
electrons resulting from the conversion of photons.

[0017] For this purpose, the method of the invention uses the data
produced by a sensor of the type comprising a vacuum chamber, a
photocathode arranged in the chamber, designed to convert the photons
emitted by at least one light source into primary electrons and subjected
to an electromagnetic field adapted to accelerate the primary electrons,
a converter converting electrons into secondary charges arranged in the
chamber facing the photocathode and adapted to convert at least part of
the energy of the accelerated primary electrons into secondary charges
collected by a plurality of detection cells distributed at a regular
pitch, the converter also converting so-called parasitic electrons not
directly derived from the conversion of a photon and possibly being of
backscattered type, of the type resulting from ion back bombardment or of
the type derived from electronic emission background noise, and an
acquisition circuit adapted to read the charges collected by the
detection cells with an integration time allowing an impact density to be
obtained per unit of time and per unit of cell surface of the order of a
single electron.

[0018] According to the invention, the method comprises the following
steps: [0019] identifying a cluster of adjacent detection cells of
which one cell called the main cell comprises a quantity of collected
charges higher than a threshold value, [0020] determining at least one
characteristic of the cluster, [0021] comparing the determined
characteristic(s) of the cluster with at least one characteristic of a
cluster resulting from the conversion of a primary electron, to evidence
a cluster resulting from the conversion of a primary electron.

[0022] According to one variant of embodiment, the method further
comprises a step to eliminate a cluster identified as not resulting from
the conversion of a primary electron.

[0023] According to one advantageous variant of embodiment, the method
further comprises a confrontation step between firstly the determined
characteristic(s) of the cluster and secondly at least one characteristic
of a cluster resulting from the conversion of a backscattered electron,
in order to identify a cluster resulting from the conversion of a
backscattered electron, and a step to eliminate clusters evidenced during
the confrontation step.

[0024] This variant advantageously allows the elimination of halo effects
due to backscattered electrons, when detecting a scene with low light
level comprising a localized more intense source.

[0025] According to one advantageous variant of embodiment, the method
comprises a step to search for similarity between firstly the determined
characteristic(s) of the cluster and secondly at least one characteristic
of a cluster resulting from ion back bombardment, in order to identify a
cluster resulting from ion back bombardment, and a step to eliminate
clusters evidenced during the confrontation step.

[0026] This variant advantageously allows the elimination of the effects
of ion back bombardment at the photocathode.

[0027] According to one advantageous variant of the embodiment, the method
also comprises the following steps: [0028] determining the position of
the photon at the origin of each identified, non-eliminated cluster,
[0029] storing at least part of all the determined positions, [0030]
comparing each new determined position with the stored positions, [0031]
identifying, from the result of comparison, the new positions which are
not included in the stored positions, in order to evidence the positions
of electrons resulting from the electronic background noise, [0032]
eliminating the new positions evidenced by the identification step.

[0033] This variant advantageously allows elimination of the effects of
electronic emission background noise derived from the emission of
electrons by the photocathode by thermionic effect or by field effect.

[0034] The method according to the invention may further comprise at least
one of the following characteristics: [0035] the identification step of
the cluster comprises a step for evidencing at least the main cell whose
quantity of collected charges is higher than a threshold value, and a
recognition step of the cluster from the charges collected in the cells
adjacent the main detection cell, [0036] the method comprises a step for
generating a video signal from the identified, non-eliminated clusters,
[0037] the method comprises a step for generating a position signal from
the determined, non-eliminated positions.

[0038] A further object of the invention is to propose a device for
detecting photons in scenes with low light level despite the presence of
parasitic effects.

[0039] For this purpose, a photon detection device according to the
invention comprises a sensor comprising a vacuum chamber, a photocathode
arranged in the vacuum chamber designed to convert photons to primary
electrons and subjected to an electromagnetic field adapted to accelerate
the primary electrons, a converter converting electrons to secondary
charges arranged in the vacuum chamber facing the photocathode and
adapted to convert at least part of the energy of the accelerated primary
electrons into secondary charges collected by a plurality of detection
cells distributed at a regular pitch, the converter also converting
so-called parasitic electrons not derived directly from the conversion of
a photon and possibly being of backscattered type, of the type resulting
from ion back bombardment or of the type derived from electron emission
background noise, and an acquisition circuit adapted to read the charges
collected by the detection cells with an integration time allowing an
impact density to be obtained per unit of time and per unit of cell
surface of the order of a single electron.

[0040] According to the invention, the detection device also comprises a
system for identifying a cluster of adjacent detection cells of which at
least one cell called the main cell comprises a quantity of collected
charges that is higher than a threshold value, a system for determining
at least one characteristic of the cluster, a system for memorizing at
least one characteristic of a cluster resulting from conversion of a
primary electron, and a system for comparing the characteristic(s) of the
cluster with the memorized characteristic(s) of a cluster resulting from
the conversion of a primary electron in order to evidence a cluster
resulting from the conversion of a primary electron.

[0041] According to one variant of embodiment, the detection device
comprises a system for eliminating a cluster identified by the comparison
system as not resulting from the conversion of a primary electron.

[0042] According to one advantageous variant of embodiment, the memory
system memorizes at least one characteristic of a cluster resulting from
the conversion of a backscattered electron, and the device comprises a
system for confronting the determined characteristic(s) of the cluster
with the memorized characteristic(s) of a cluster resulting from the
conversion of a backscattered electron in order to evidence a cluster
resulting from the conversion of a backscattered electron, and the device
comprises a system for eliminating a cluster evidenced by the confronting
system.

[0043] This variant advantageously prevents the formation of a halo effect
in a scene with low light level comprising a localized more intense
source.

[0044] According to one advantageous variant of embodiment, the memory
system memorizes at least one characteristic of a cluster resulting from
ion back bombardment, and the device comprises a system for confronting
the determined characteristic(s) of the cluster with the memorized
characteristic(s) of a cluster resulting from ion back bombardment in
order to evidence a cluster resulting from ion back bombardment, and the
device comprises a system for eliminating a cluster evidenced by the
confronting system.

[0045] This variant advantageously allows elimination of the effects of
ion back bombardment at the photocathode.

[0046] According to one advantageous variant of embodiment, the device
further comprises a system for determining the position of the photon at
the origin of each identified, non-eliminated cluster, a system for
storing at least part of all the determined positions, a system for
comparing each new determined position with the stored positions, a
system using the result of comparison to identify new positions which are
not included in the stored positions, in order to evidence the positions
of electrons resulting from electronic emission background noise by
thermionic effect or by field effect, and a system for eliminating the
positions evidenced by the identification system.

[0047] This variant advantageously allows the elimination of the effects
of electronic emission background noise derived from the emission of
electrons by the photocathode under thermionic effect or by field effect.

[0048] The device of the invention may further comprise at least one of
the following characteristics: [0049] the identification system of the
cluster comprises a system for identifying at least one main cell whose
quantity of collected charges is higher than a threshold value, and a
system for recognizing the cluster from the collected charges in the
cells adjacent the main detection cell, [0050] the device comprises a
system for generating a video signal from identified, non-eliminated
clusters during a given time interval, [0051] the device comprises a
system for generating a position signal from each determined,
non-eliminated position.

[0052] Various other characteristics will become apparent from the
description given below with reference to the appended drawings which, as
non-limiting examples, illustrate embodiments of the subject of the
invention.

[0053]FIG. 1 is a diagram showing an example of embodiment of a device
according to the invention.

[0054]FIG. 2 shows an example of a cluster resulting from the conversion
of a primary electron, and the quantities of charges collected in the
cells of the cluster.

[0059] The photon detection device 1 shown FIG. 1 is adapted for scenes
with low light level. This detection device 1 comprises a sensor 2 which
is built with single-photon detection sensitivity. In the illustrated
preferred embodiment, this sensor 2 is of EBCMOS type. Evidently the
invention is not limited to devices provided with a sensor of this type.
Any silicon sensor capable of single-photoelectron sensitivity such as
hybrid pixels for example or CMOS sensors of SOI type
(Silicon-On-Insulator) can be used under the invention.

[0060] The device 1 comprises a given vacuum chamber 3 and a photocathode
4 arranged in the chamber 3. The photocathode 4 is adapted to convert
photons 5 emitted by at least one light source, not illustrated, into
electrons. The group of photons emitted by the source(s) and converted at
least in part by the device defines an incident flow. In the present
description, the electrons resulting from the conversion of a photon 5 by
the photocathode 4 are called primary electrons 6.

[0061] The photons 5 can derive from the visible spectrum and/or near
infrared and/or near ultraviolet for example.

[0062] The photocathode 4 is subjected to an electromagnetic field E
induced by a potential difference and set up by means of a system 7 for
generating the electromagnetic field E. The said electromagnetic field E
is adapted to accelerate the primary electrons 6 from the photocathode 4
as far as an electron converter 8, so as to generate impacts of primary
electrons 6 on the electron converter. The photocathode 4 lies distant
from the electron converter 8 by a separating distance D.

[0063] The value of the electromagnetic field E is adapted to impart
sufficient kinetic energy to the primary electrons 6 to allow the
individual detection of each primary electron 6 by the sensor 2.

[0064] The electron converter 8 is adapted to convert at least part of the
energy of the accelerated primary electrons 6 into secondary charges 9
collected by a plurality of detection cells 10 distributed at a regular
pitch, for example in an array.

[0065] In the illustrated example of embodiment, the electron converter 8
comprises a so-called passive input layer 11, having a thickness e,
arranged facing the photocathode 4 and through which at least part of the
primary electrons 6 pass. The electron converter 8 also comprises a
detection volume 12, adjacent the input layer 11, in which at least the
primary electrons 6 interact to form electron-hole pairs which diffuse as
far as the detection cells 10, preferably diodes arranged in an array.

[0066] Preferably the electron converter 8 is a CMOS component
(Complementary Metal-Oxide Semiconductor) and of MAPS type (Monolithic
Active Pixel Sensor). Evidently, other types of primary electron
converters 8 can be used under the invention such as an electron
converter for example provided with a sensitive layer connected via beads
or TSVs (Through Silicon Via) to a CMOS reading circuit.

[0067] The device 1 further comprises an acquisition circuit 13 adapted to
read out the charges 9 collected by the detection cells 10 with an
integration time adapted to obtain an impact density on the electron
converter 8 per unit of time and per unit surface of a cell 10 of the
order of a single electron. The integration time is preferably equal to
or less than 1 ms.

[0068] According to one advantageous variant of embodiment, the
integration time is calculated, whether or not dynamically, by a system
computing the integration time, not illustrated, in relation to the size
of the detection cells 10 and the incident light flux.

[0069] The acquisition circuit 13 is adapted to offer a ratio between the
read-out charges 9 and a read-out noise generated on reading the
detection cells 10, adapted to allow detection sensitivity to
single-photons 5. The ratio between the read-out charges 9 and read-out
noise is dependent at least on the value of the electromagnetic field E,
on the thickness e of the passive layer of the electron converter 8 and
on the separating distance D between the photocathode 4 and the electron
converter 8.

[0070] According to the invention, the device 1 further comprises an
identification system 14 to identify a cluster 15 of which one example is
illustrated FIG. 2. Here, FIG. 2 illustrates a cluster 15 resulting from
the conversion of a primary electron.

[0071] The identification system 14 identifies a cluster 15 of adjacent
detection cells 10 of which at least one so-called main cell 10a contains
a quantity of collected charges 9 that is higher than a threshold value
Vs, from the charges 9 read by the acquisition circuit 13.

[0072] Preferably, the chosen threshold value Vs is dependent on the value
of the read-out noise of the detection cell 10. The threshold value Vs is
preferably equal to 5 times the value of the read-out noise.

[0073] According to the illustrated preferred embodiment, the
identification system 14 comprises a system 17 for evidencing at least
the main cell 10a whose quantity of collected charges 9 is higher than
the threshold value Vs. The identification system 17 further comprises a
recognition system 18 recognizing the cluster 15 from the quantities of
charges 9 collected in the cells 10b adjacent the main cell 10a.

[0074] The optimal size of the cluster 15, i.e. the number of adjacent
cells 10 taken into account, is variable and is dependent upon the
distribution of charges 9 around the main cells 10a and on the pitch of
the detection cells 10. Preferably, the size of the cluster 15 is
3×3, 5×5 or 7×7 detection cells 10. In the illustrated
example of embodiment, the size of the cluster 15 is 3×3 detection
cells 10.

[0075] The optimal size of the cluster 15 may advantageously be computed,
whether or not dynamically, by a system for determining the optimal
cluster size, not illustrated.

[0076] Advantageously, the identification system 14 identifying a cluster
15 allows the identification of regions potentially corresponding to an
impact of a primary electron 6 on the electron converter 8 with a view to
analysis thereof.

[0077] The device 1 further comprises a determination system 19
determining at least one characteristic of the cluster 15.

[0078] Preferably, the determination system 19 determines the total
quantity of charges 9 collected in the cluster 15 by summing the charges
9 collected in the main cells 10a and adjacent cells 10b of the cluster
15.

[0079] According to different variants of embodiment, the determination
system 19 is able to determine other characteristics of the cluster 15
such as its topology for example or its mean density of charges 9.

[0080] For example the cells 10a, 10b whose quantity of collected charges
9 is the highest may form a cross- or square-shaped pattern depending on
whether the impact of the electron at the origin of the cluster 15 lies
respectively above a single cell 10a, 10b or between 4 cells 10a, 10b.

[0081] The device 1 further comprises a memory system 20 to memorize at
least one characteristic of a reference cluster 15a resulting from the
conversion of a primary electron 6. The characteristics of the reference
cluster 15a are known per se, and the choice of characteristic(s) is
purely arbitrary depending on the embodiment of the device 1.

[0082] Preferably, the memory system 20 memorizes the total quantity of
charges 9 of the reference cluster 15a which ranges from 220 to 280 Qadc
and is preferably 250 Qadc for a mean noise per cell of 3 Qadc.

[0083] According to one variant of embodiment, the memory system 20
memorizes the mean density of charges 9 of the reference cluster 15a
which ranges from 24 to 30 and is preferably 27 Qadc/cell for an array of
3×3 cells 10 with a pitch of 17 μm.

[0084] The device 1 further comprises a comparison system 21 comparing the
determined characteristic(s) of the cluster 15 with the memorized
characteristic(s) of the reference cluster 15a, in order to evidence
whether the cluster 15 results from the conversion of a primary electron
6. In other words, the cluster 15 is compared with the reference cluster
15a which has a known profile, so as to determine whether these clusters
15, 15a are similar.

[0085] Preferably, the comparison system 21 compares the total quantity of
charges 9 collected in the cluster 15 with the total quantity of charges
in the reference cluster 15a.

[0086] The device 1 of the invention thereby allows simple reliable
identification of the primary electrons resulting from photon conversion.

[0087] The device 1 allows an image and/or a video signal to be generated,
formed of a succession of images, each image being generated from the sum
of the charges 9 collected by the detection cells 10 during the
integration time of the sensor 2.

[0088] However it is to be noted that the electron converter 8 also, and
which is undesired, converts so-called parasitic electrons into secondary
charges 9, these parasitic electrons not deriving from the conversion of
a photon 5. The parasitic electrons may be at least of backscattered
type, of the type resulting from ion back bombardment or of the type
derived from electronic emission background noise. Each type of parasitic
electron generates a different type of parasitic effect. The parasitic
electrons of backscattered type generate a halo parasitic effect, the
parasitic electrons of the type resulting from ion back bombardment
generate a so-called "ion back bombardment" parasitic effect, and
parasitic electrons of the type derived from electron emission background
noise by the photocathode generate a so-called "dark count" parasitic
effect.

[0089] Therefore, it is advantageous to identify and preferably eliminate
the clusters 15 resulting from conversion of parasitic electrons so as
only to keep those clusters 15 resulting from the conversion of primary
electrons 6 in order to generate an image and/or video signal without
parasitic effects.

[0090] According to one variant of embodiment, any cluster 15 whose
determined characteristics have at least one difference compared with the
memorized characteristic(s) of the reference cluster 15a is considered to
be a cluster resulting from the conversion of a parasitic electron and is
therefore eliminated. The device 1, according to this variant of
embodiment, comprises an elimination system 22 connected to the output of
a comparison system 21 and adapted to eliminate those clusters 15
identified by the comparison system 21 as not resulting from the
conversion of a primary electron.

[0091] According to one advantageous variant of embodiment, the device 1
allows the identification of at least one and preferably of the three
types of parasitic electrons defined above, for example with a view to
elimination thereof.

[0092] As explained in FIG. 3, the halo effect is due to electrons 61
backscattered by the electron converter 8 i.e. scattered by the electron
converter in the direction of the photocathode 4. Between 12% and 18% of
the primary electrons 6 are backscattered by the electron converter 8,
accelerated by the electromagnetic field E in the direction of the
photocathode 4 then detected at a position different from the position of
the initial primary electron 6. It is possible for example, using
simulations performed using the Monte-Carlo method or using analytical
calculations, to show that the distance R travelled by a backscattered
electron between the primary impact and the secondary impact is equal to
no more than twice the distance between the photocathode 4 and the
electron converter 8.

[0093]FIG. 4A shows an image of a scene with low light level i.e. less
than one mLux, and containing a localized more intense source formed of
an optical fibre 23. This image derives from a usual EBCMOS sensor 2
according to the state of the art. The accumulation of charges 9, due to
the backscattered electrons 61, leads to the formation of a circular
halo 24 around the more intense source. All the images of the objects
contained in the halo 24, for example an object 25, are attenuated and/or
masked by the halo 24.

[0094] According to one embodiment of the invention advantageously adapted
to eliminate the halo effect, the memory system 20 memorizes at least one
characteristic of a standard cluster 15b resulting from the conversion of
a backscattered electron 61.

[0095] The characteristics of the standard cluster 15b are known per se,
and the choice of characteristic(s) is purely arbitrary and depends upon
the embodiment of the device 1.

[0096] Preferably the memory system 20 memorizes the total quantity of
charges 9 of the standard cluster 15b which ranges from 75 to 125 Qadc
and is preferably 125 Qadc.

[0097] According to this variant, the device 1 also comprises a
confrontation system 26 confronting the determined characteristic(s) of
the cluster 15 with the memorized characteristic(s) of the standard
cluster 15b in order to evidence whether the cluster 15 results from the
conversion of a backscattered electron 61. In other words, the
cluster 15 is compared with the standard cluster 15b which is of known
profile, so as to determine whether these clusters 15, 15b are similar.

[0098] Preferably, the confrontation system 26 confronts the total
quantity of charges 9 collected in cluster 15 with the total quantity of
charges 9 in the standard cluster 15b. According to this variant, the
device 1 preferably comprises an elimination system 12 to eliminate the
cluster 15 corresponding to a backscattered electron 61 evidenced by
the confrontation system 26.

[0099]FIG. 4B illustrates a similar image to the image in FIG. 4A but
obtained with a device 1 of the invention adapted to eliminate halo
effects. The images of the objects close to the light source 23, for
example object 25, are not masked.

[0100] The effect of ion back bombardment at the photocathode 4 is due to
ionization by the primary electrons 6 of a residual atom present in the
chamber 3. Once positively ionized, the residual atom is accelerated by
the electromagnetic field E in the direction of the photocathode 4. This
results in localized pulling away of numerous electrons from the
photocathode 4, these pulled electrons themselves being accelerated in
the direction of the electron converter 8.

[0101] According to one embodiment of the invention advantageously adapted
to eliminate the effect of ion back bombardment, the memory system 20
memorizes at least one characteristic of a model cluster 15c resulting
from ion back bombardment. The characteristics of the model cluster 15c
are known per se and the choice of characteristic(s) is purely arbitrary
and depends on the embodiment of the device 1. Preferably, the memory
system 20 memorizes the total quantity of charges 9 of the model cluster
15c which ranges from 1000 to 4000 Qadc and is preferably 2000 Qadc.
According to one variant of embodiment, the memory system 20 memorizes
the mean density of charges 9 of the model cluster 15c which lies between
40 and 60 Aadc/cell and is preferably 50 Qadc/cell in an array of
7×7 cells 10 with a pitch of 17 μm.

[0102] According to this variant, the device 1 further comprises a
similarity search system 28 between the determined characteristic(s) of
cluster 15 and the memorized characteristic(s) of the model cluster 15c,
in order to evidence whether cluster 15 results from ion back
bombardment. In other words, the similarity search system 28 carries out
a comparison between cluster 15 and the model cluster 15c which is of
known profile so as to determine whether these clusters 15, 15c are
similar. Preferably, the similarity search system 28 confronts the total
quantity of charges 9 collected in cluster 15 with the total quantity of
charges 9 in the model cluster 15c.

[0103] The device 1 preferably comprises an elimination system 22 to
eliminate a cluster 15 considered as resulting from ion back bombardment
and evidenced by the similarity search system 28.

[0104] The effect of electronic emission background noise is due to
electrons spontaneously and randomly emitted by the photocathode 4 under
thermionic effect or field effect. For images of scenes with low light
levels, this type of effect translates as a random "snow" effect in the
image. Advantageously, it is possible to detect the electrons of the type
derived from electronic emission background noise by determining whether
or not they are randomly localized.

[0105] For this purpose the device 1, according to one variant of the
invention adapted to eliminate the effects of electronic emission
background noise, comprises a determination system 29 to determine the
position of the photon 5 at the origin of each identified and
non-eliminated cluster 15. According to this variant, the device 1
further comprises a storage system 30 to store at least part of all the
determined positions, and a comparison system 31 comparing each new
determined position with the stored positions. The determination system
29 and the storage system 30 are therefore connected to the comparison
system 31.

[0106] The device 1 preferably comprises an identification system 32 using
the result of the comparison of the new positions which are not included
in the stored positions, in order to evidence the positions of photons
resulting from electronic emission background noise.

[0107] In other words, the identification system 32 determines whether
each new position is already included in the stored positions, in which
case the new position probably corresponds to a photon 5 emitted by one
of the light sources, or whether it is not included in the stored
positions in which case it probably corresponds to the conversion of an
electron of the type derived from electronic emission background noise by
the electron converter 8.

[0108] The device 1 also preferably comprises an elimination system 22 to
eliminate those positions evidenced by the identification system 32.

[0109] Evidently, the positions eliminated by the elimination system 32
may be stored by the storage system 30 for the purpose of being compared
with subsequent positions. Advantageously, this characteristic prevents
the elimination of photons from an apparent light source.

[0110] It is to be noted that the embodiment adapted to eliminate the halo
effect, the embodiment adapted to eliminate the ion back bombardment
effect and the embodiment adapted to eliminate electronic emission
background noise are not incompatible. A device 1 according to the
invention is able to implement one and/or the other of these embodiments
in combination, and preferably all three.

[0111] Finally, the device 1 comprises a system 33 for generating an
output signal. According to one variant of the invention, this signal is
a video signal composed of a plurality of images, and the system 33
generating an output signal comprises a system for generating a video
signal from identified and non-eliminated clusters during a given time
interval, for example 40 ms, so as to obtain an image frequency of 25 Hz.

[0112] According to one variant of the invention, this signal is a digital
signal giving the positions of the detected photons. In this case, the
system 33 for generating a signal is composed of a system for generating
a position signal from each determined and non-eliminated position.

[0113] Advantageously, this variant allows the subsequent reconstitution
at will of an image by an external processing system, not illustrated,
from the successive positions of the photons.

[0114] According to one variant of embodiment it is to be noted that,
using the position signal, it can be envisaged to determine the position
of a point emitter of single photons per image and optionally to track
the position of this point emitter.

[0115] The method of the invention, of which the logical diagram is given
FIG. 5, processes the data produced by a sensor 2 built for single-photon
sensitivity, for the purpose of identifying primary electrons derived
from the conversion of photons to allow the optional overcoming of at
least one type of parasitic effect when processing scenes with low light
levels.

[0116] These parasitic effects may be of several types which include a
halo parasitic effect, a so-called "ion back bombardment" parasitic
effect and a so-called "dark count" parasitic effect.

[0117] In this example of embodiment and preferably, the method of the
invention is designed for implementation on the device 1 according to the
invention and/or to process data derived from a sensor 2 according to the
invention. Evidently, it is possible to implement said method on a device
not conforming to the subject of the invention.

[0118] The method comprises a first step E1 to identify a cluster 15 of
adjacent detection cells 10 of which at least one so-called main cell 10a
comprises a quantity of collected charges 9 that is higher than a
determined threshold value Vs. In the illustrated example of embodiment,
the identification of the cluster 15 is made by means of the
identification system 14.

[0119] According to one variant of embodiment, step E1 comprises a first
identification sub-step E11 to identify at least the main cell 10a whose
quantity of collected charges 9 is higher than the threshold value Vs,
and a second sub-step E12 for recognition of the cluster 15 from the
quantities of collected charges 9 in the cells 10b adjacent the main cell
10a. In the illustrated example of embodiment, the identification of the
main cell 10a is performed by means of the evidencing system 17 and
recognition of the cluster 1 is performed by the recognition system 18.

[0120] Next, at a second step E2, the method determines at least one
characteristic of the cluster 15. In the illustrated example of
embodiment, the determination is performed by means of the determination
system 19. Preferably the second step E2 determines the total quantity of
charges 9 collected in the cluster 15 by summing the charges 9 collected
in the different cells 10a, 10b of the cluster 15.

[0121] The method, at a third step E3, then compares the determined
characteristic(s) of the cluster 15 with at least one characteristic of a
reference cluster 15a resulting from the conversion of a primary electron
6, in order to evidence whether the cluster 15 results from the
conversion of a primary electron 6. In other words, the method compares
cluster 15 with the reference cluster 15a of known profile so as to
determine whether these clusters 15, 15a are similar.

[0122] The characteristics of the reference cluster 15a are known per se
and the choice of characteristic(s) is purely arbitrary and is dependent
upon the embodiment of the method of the invention. In the illustrated
example of embodiment, the comparison performed by the comparison system
21. Preferably, the third step E3 compares the total quantity of charges
9 collected in cluster 15 with the total quantity of charges 9 in the
reference cluster 15a.

[0123] Preferably, at least the first, the second and the third step are
successively and continuously repeated.

[0124] The method of the invention therefore allows the detection of
photons through the identification of those clusters resulting from the
conversion of primary electrons.

[0125] According to one variant of embodiment, not illustrated, it is to
be noted that the method can implement a step to eliminate clusters
identified as not resulting from the conversion of primary electrons.

[0126] According to one variant advantageously adapted to eliminate halo
effects, the method comprises a fourth step E4 to confront the determined
characteristic(s) of the cluster 15 with at least one characteristic of a
standard cluster 15b resulting from the conversion of a backscattered
electron 61 in order to identify whether the cluster 15 results from
the conversion of a backscattered electron 61. The method therefore
compares the cluster 15 with the standard cluster 15b which is of known
profile so as to determine whether these clusters 15, 15b are similar.

[0127] The characteristics of the standard cluster 15b are known per se,
and the choice of characteristic(s) is purely arbitrary and is dependent
upon the embodiment of the method of the invention.

[0128] Preferably, the fourth step E4 compares the total quantity of
charges 9 collected in the cluster 15 with the total quantity of charges
9 in the standard cluster 15b. In this example, confrontation is
performed by means of the confrontation system 26. According to this
variant, the method further comprises a fifth step E5 to eliminate the
clusters 15 evidenced at the fourth step E4. In this example of
embodiment, elimination is performed by means of the elimination system
22.

[0129] According to one variant advantageously adapted to eliminate the
effects of ion back bombardment, the method further comprises a sixth
step E6 to confront the determined characteristic(s) of cluster 15 with
at least one characteristic of a model cluster 15c resulting from ion
back bombardment, in order to identify whether the cluster 15 results
from ion back bombardment. The method therefore compares cluster 15 with
the model cluster 15c which is of known profile, so as to determine
whether these clusters 15, 15c are similar.

[0130] The characteristics of the model cluster 15c are known per se and
the choice of characteristic(s) is purely arbitrary and is dependent upon
the embodiment of the method of the invention. Preferably, the sixth step
E6 compares the total quantity of charges 9 collected in cluster 15 with
the total quantity of charges 9 in the model cluster 15c. In the
illustrated example of embodiment, confrontation is performed by means of
the similarity search system 28.

[0131] In this variant, the method further comprises a seventh step E7 to
eliminate the clusters 15 evidenced at the sixth step E6. In the
illustrated example of embodiment, elimination is performed by means of
the elimination system 22.

[0132] According to one variant advantageously adapted to eliminate the
effects of electronic emission background noise, the method further
comprises an eighth step E8 to determine the position of the photon 5 at
the origin of each cluster 15 that is identified and non-eliminated. In
the illustrated example, the determination of the position of the photon
5 is performed by means of the determination system 29 determining the
position of the photon 5.

[0133] The method further comprises a ninth step E9 to store at least part
of all the determined positions, performed in this example by means of
the storage system 30 of the device 1. According to this variant, each
new determined position is compared with the positions stored at a tenth
step E10, and the new positions which are not included in the stored
positions are identified from the result of the comparison at an eleventh
step E11 to evidence the positions resulting from electronic emission
background noise.

[0134] In the illustrated example of embodiment, the comparison is
performed by means of the comparison system 21 of the device and the
identification of the new positions is performed by the identification
system 32. In addition, this variant of embodiment comprises a twelfth
step E12 to eliminate the new positions evidenced at the eleventh step
E11. In this example of embodiment, elimination is performed by means of
the elimination system 22.

[0135] Evidently the eliminated positions can nevertheless be stored for
subsequent comparison with new positions, so that it is possible to
identify the onset of a new light source whose photons 5 must not be
eliminated.

[0136] It is to be noted that the variant of the method adapted to
eliminate halo effects, the variant of the method adapted to eliminate
the effects of ion back bombardment, and the variant of the method
adapted to eliminate the effects of electron emission background noise
are not incompatible. A method according to the invention is able to
implement one and/or the other of these variants and preferably all
three.

[0137] Finally the method of the invention comprises a thirteenth step E13
to generate an output signal.

[0138] According to one variant of the invention, this signal is a video
signal composed of a plurality of images and the thirteenth step E13
consists of generating a video signal from identified and non-eliminated
clusters 15 over a given time interval, for example 40 ms, so as to
obtain an image frequency of 25 Hz.

[0139] According to another variant of the invention, this signal is a
digital signal giving the positions of the detected photons 5, and the
thirteenth step E13 consists of generating a position signal from each
determined, non-eliminated position. On the basis of this step to
generate an output signal, the method of the invention according to one
variant of embodiment is able to determine the position of a
single-photon point emitter per image. The method can be adapted to track
the position of this point emitter.

Patent applications by CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE

Patent applications by UNIVERSITE CLAUDE BERNARD LYON I

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